Manufacturing method of high tension, high toughness steel

ABSTRACT

Method for obtaining a recrystallized austenite structure in steel by carrying out more than one severe reduction per pass between 1,100*C and 950*C in the hot rolling process of a steel comprising 0.005 - 0.15% C, less than 0.6% Si, 0.70 - 2.0% Mn and 0.01 - 0.15% Nb and containing other effective elements if necessary, with the remainder being essentially Fe, and then rolling the steel to obtain a high toughness, high tension steel plates and strips.

Ishizaki et al.

111 3,849,209 Nov. 19,1974

MANUFACTURING METHOD OF HIGH TENSION, HIGH TOUGHNESS STEEL Inventors: Keizo Ishizaki, Kitakyoshu; Hiroshi Sekine, Tokyo; Hisashi Gondo, Hisashi; Koji Wada, Kisarazu; Tsuyoshi Kawano, Kisarazu; Hidekazu Watanabe, Kisarazo; Gou Kanbayashi, Chiba-ken; Tadasatsu Maruyama, Tokyo, all of Japan Assignee: Nippon Steel Corporation, Tokyo,

Japan Filed: Oct. 11, 1973 App]. No.: 405,635

Related US. Application Data Continuation-impart of Ser. No. 222,496, Feb. 1, 1972, abandoned.

[56] References Cited UNITED STATES PATENTS 3,328,211 6/1967 Nakamura 148/12 F 3,645,801 2/1972 Melloy et al. 148/12 F Primary ExaminerW. Stallard Attorney, Agent, or FirmToren, McGeady and Stanger 57 ABSTRACT Method for obtaining a recrystallized austenite structure in steel by carrying out more than one severe reduction per pass between l,lOC and 950C in the hot rolling process of a steel comprising 0.005 0.15% C, less than 0.6% Si, 0.70 2.0% Mn and 0.01 0.15% Nb and containing other effective elements if necessary, with the remainder being essentially Fe, and then rolling the steel to obtain a high toughness,

u s c1 148/12 F o o o a a a a a a a a 4 a t 4 a s u l h S. 1111.01. C2ld 7/12, C2ld 7/14 g p Field of Search 148/12 F 5 Clalms, 2 Drawmg Flgures o+1 2 i l l 0 1 2 Austenite Grain I Size Number L- 4 1 l l Q 6 Non-recr stcllized l o Auslenite 8 Reduction Temperature vplute Thickness 7Omn160m1in 50mm lAOmmISOmm |20mm l 14mm I 10mm [7mm Redum 14% 17-/. 20v. 33-1., v. 29v. 30%

Temperature C FIG.2

Plate Thickhss=117mm vTrs (C) Coiling Tempercture(C) MANUFACTURING METHOD OF HIGH TENSION, HIGH TOUGHNESS STEEL CROSS REFERENCES TO RELATED APPLICATIONS This application is a continuation-in-part of copending application Ser. No. 222,496, filed Feb. 1, 1972, and now abandoned, the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION This invention relates to a manufacturing method for hot rolled steel material containing Nb (Nb-steel) having high strength, high toughness and excellent weldability as a high tension, high toughness steel, particularly as a structural steel material having a thickness of more than 6 mm or desirously more than 9 mm.

It has been well known that (1) structural Nb-steel is used as a high tension steel because its strength is increased by the precipitation of NbC in the ferrite matrix during cooling after transformation; (2) it is important in the preparation of such steel, to keep the temperature of the slab high during the heating before the hot rolling in order to utilize the Nb effectively in the precipitation hardening, and (3) the elevation of strength causes, in many instances, the lowering of the toughness, in general, and particularly, the Nb-steel heated above 1,200C, the toughness decreases rapidly as the thickness of the steel is increased. As a countermeasure against the lowering of the toughness, many investigations have been conducted to improve the toughness by controlling the heating and rolling conditions. For instance, a method has been proposed in J apanese Patent Publication, Sho 40-28241, in which a slab is heated below 1 ,190C before hot rolling and the final rolling is carried out below l,000C. The method proposed in Japanese Patent Publication, Sho 44-27139 is that, after heating a slab containing Nb alone above l,230C, a severe working with a total reduction of more than 50 percent of the original plate thickness is carried out in the temperature region higher than l,lC, and the rolling is continued in the temperature range between 850C and 700C, the reduction being more than 30 percent of the plate thickness at 850C. 7

While various methods have been tried to improve the toughness of Nb-steel as in the above publication, these methods have the following disadvantages. in the low temperature heating method, as in the first example, since the toughness is obtained by sacrificing the ing is used, the strength becomes higher than in the low temperature heating material, but a severe integrated reduction is necessary in the low temperature region (below 850C) to ensure toughness.

However, a severe integrated reduction at a low.temperature which is widely adopted in the usual processes frequently makes the shape of the rolled steel product inferior. Particularly, in order to carry out the final stage of rolling with a defined amount of reduction at a defined low temperature region in a continuous rolling process, for a steel with a thickness of more than 9 mm, it is necessary to suspend the successive reductions in the course of rolling to lower the temperature of the steel, thus resulting also in a lowering of the productivity. Moreover, as is shown in the specification of Japanese Patent Publication, Sho 44-27 l 37, the partial recrystallization state of austenite grains is caused by the effect of Nb addition which retards the recrystallization of austenite due to inadequate selection of the temperature region of the suspension of rolling. Moreover, this heterogeneous state does not disappear in the subsequent rolling, leaving coarse ferrite grains or an upper bainite structure in the steel material after transformation and thus substantially lowering the toughness. Thus, in order to obtain a satisfactory product in the factory operation, an enormous trial and error method is required for every composition.

SUMMARY OF THE INVENTlON As a consequence of extensive investigations on the behavior of Nb-steel during hot rolling, the development of an excellent hot rolled steel plate and strip has been made in the present invention by adopting a new method of rolling which has never been considered.

The object of this invention is to offer a manufacturing method of Nb-steel at a very high productivity as compared with conventional methods and allowing consistent production of a product with strength,

toughness and weldability, far beyond the level of conventional steels as rolled.

Another object of this invention is to offer a-Nb-steel having particularly excellent toughness in the rolled condition.

Other objects of this invention will be clear from the following description and the attached drawings.

- The main features of this invention lie in a manufacturing method of a high tension, high toughness steel characterized in which:

A slab comprising C 0.005 0.15 percent (desirously 0.01 0.12

percent) Se less than 0.60 percent Mn 0.70 2.0 percent and Nb 0.01 O.l5 percent, if necessary at least one metal selected from the group consisting of V less than 0.20 percent and Al less than 0.08 percent,

and further, if necessary also at least one metal selected from the group consisting of Ni less than 1.00 percent Cr less than 1.00 percent Cu less than 1.00 percent Ti less than 0.20 percent and Zr less than 0.20 percent, with the remainder beingiron and unavoidable impurity elements is l. subjected to hot roll after heating to a temperature higher than 1,200C, rolled under conditions wherein at least one severe reduction with a draft of more than 26 percent per one drawing is effected in a temperature region between l,l00C and 950C (which will be mentioned as the high temperature region hereinafter) and the total reduction thereafter below 950C down to the finish of the rolling (which will be mentioned as the low temperature) becomes more than 45 percent of the plate thickness at 950C, and the rolling is finished in the temperature range between 850C and the Ar transformation point; or

2. in step (I), the total reduction below 850C is defined to be less than 30 percent of the plate thickness at 850C; or

3. in step 1 the steel plate and strip after rolling is coiled in the temperature range between 680C and 500C; or

4. in step (1), the steel plate and strip after rolling is coiled in the temperature range above 700C; or further 5. in step (2), the steel plate and strip after rolling is coiled in the temperature range between 680C. and 500C.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing the existence of completely recrystallized austenite and the grain size of the recrystallized austenite immediately before each reduction for various plate thicknesses and various rolling temperatures for an inventive steel (0.10% C, 0.3% Si, 1.5% Mn, 0.06% Nb and 0.006% N) heated at 1,250C and rolled from 1,150C.

FIG. 2 is a graph showing the relationship between the coiling temperature and the mechanical properties (yield point and low temperature toughness).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT After a result of various investigations, the present inventors have discovered the following points.

The first point is that the material retarding the recrystallization of austenite in the Nb-steel during hot rolling is NbC and NbN which are dissolved in austenite by heating of a slab and are precipitated as fine crystals in the course of rolling. While the NbCand NbN are effective to make the grain size of the steelfine, they have almost no effect on the precipitation hardening of steel. When the rolling is done at the same amount of total reduction, the said precipitation of NbC and NbN during hot rolling is accelerated as the rolling temperature is lowered. Namely, it was found experimentally that, when a steel containing 0.10% C, 0.3% Si, 1.5% Mn, 0.06% Nb and 0.006% N was hot rolled as in FIG. 1 in the temperature region between 1,150C and 950C, the amount of Nb precipitated during the period from the commencement of the hot rolling to the appearance of ferrite was about 0.01 percent. It was ascertained also that, when the rolling was carried out from 950C to 830C under the same reduction schedule, the amount was nearly 0.02 percent, corresponding to one third of the total addition amount. As a result, the strength of the as-rolled Nb-stee1 with the composition shown in Table l is, as shown in Table 2, distinctly higher when the hot rolling is finished at a higher temperature than when the hot rolling is finished at a lower temperature. The tendency is more distinct in a high Nb-steel. This shows that, in order to obtain a high strength in the case of Nb-steel, a higher temperature rolling is recommendable.

The second is that, while the grain size of austenite in a slab becomes coarse during heating, austenite recrystallizes as fine new cystal grains in the course of the early stage of hot rolling. The higher the amount of each reduction and the finer the austenite grains before each reduction, the recrystallization can proceed down to the lower temperature region, and the recrystallized grains themselves become finer. When the rolling is commenced immediately after the heating of a slab as in the practical rolling procedure in the factory, austenite becomes successively finer at each reduction due to the rolling and recrystallization. 1n the case of Nb-steel, and particularly as in the present invention, when lowcarbon or low-nitrogen steel is rolled at 1,100C 950C under one or more severe drawings, even when the heating temperature is as high as 1,250C, the recrystallization of austenite is completed smoothly giving finer grains at a temperature lower than 1,050C, this temperature has hitherto been considered to be unsuitable for the recrystallization of austenite, for instance, 7 seconds after the rolling at 950C as shown in FIG. 1. It is a well known fact that to continue the rolling in the temperature region where no recrystallization of austenite proceeds is effective for the improvement of the toughness of steel. However, when sufficiently fine recrystallized austenite can be obtained by the rolling down to 950C as in the method of this invention, the steel plate and strip whose rolling is finished at this condition possesses, as shown in Table 2, high toughness after cooling.

Table 1 Steel C Si Mn Nb (71) Table 2 Steel slab Temperature of severe reductions Finish thickbetween 1 C 950C and temperaness(mm) plate thickness ture 970C A 90 mm 75 mm 60 mm 830C (no severe drawing more than 26%) 105 990C 30 mm 20 mm 950C 1020C B 70 30 mm 20 mm 900C 33% 1030C 990C 70 30 mm 20 mm 14 mm 990C Finish Yield Tensile ElongavE vTr, thickpoint strength tion (kg-mlcm (C) ness (kg/mm) (kg/mm) (71) Heating temperature of slabs: 1250C. Rolling temperature was measured by a thermocouple buried in the slab. for a subsize test piece with a thickness of 5mm.

The third point is as follows. When the mean volume of the austenite grains is the same, ferrite grains transformed from unrecrystallized austenite grains become generally far finer than that transformed from recrystallized or polygonal austenite grains. This is because many deformation bands exist in unrecrystallized austenite grains, and while ferrite nuclei are formed only at the boundaries of the austenite grains in the transformation from recrystallized austenite, ferrite nuclei are formed not only at the boundaries of austenite grains but also at the boundaries of said deformation bands at the time of the transformation from unrecrystallized austenite, thereby giving finer ferrite. However, in the case of this invention, since the grain size of recrystallized austenite just before the recrystallization stops is very fine as compared with that obtained in conventional methods, although the density of the deformation bands in uncrystallized austenite grains is smaller than those usually obtained, uniform and very fine ferrite grains can be obtained, and the content of upper bainite in the steel after rolling can also be reduced remarkably.

In other words, a sufficiently high toughness can be obtained constantly by performing one or more severe reductions in the specified high temperature region as above mentioned in this invention, even though the amount of a total reduction in the low temperature region is small and the finish temperature is high as compared with the conventional case.

The fourth is that, for the steel grade whose strength is expected to be improved by the precipitation particularly of NbC, VN and other compounds during cooling, the transformation point should be kept low in order to ensure the high strength and toughness.

The decreased amount of C, although it elevates the transformation temperature, at the same time, it elevates the upper limit of Nb which can exist in a solid solution of austenite at the soaking temperature. It is further effective to enlarge the temperature range of rolling where the recrystallization of austenite can finish during successive passes toward the low temperature side, so long as the content of Nb is the same. Therefore, the addition or the increase of the amount of such elements as Mn, Ni and Cr, which lower the nonequilibrium transformation temperature during cooling effectively, is effective not only for improving the toughness due to the restriction of over-aging of NbC and other compounds precipitated in the ferrite matrix, but also for diminishing the plate thickness dependencies of strength and toughness. The rapid cooling after rolling also is particularly easy in the continuous hot strip rolling process, and the lowering of the transorrnation temperature by the rapid cooling is effective for improving the strength and toughness.

Finally, as the fifth point, it is proved by our further experiments that, by coiling at a high temperature above 700C and cooling slowly after coiling, ferrite grains remain sufficiently fine, though the strength is somewhat decreased, due to the over aging of NbC and NbN, thus giving a steel material possessing excellent toughness.

According to the various discoveries as above mentioned, we have succeeded in this invention to obtain recrystallized austenite grains finer than ever by applying, quite apart from conventional ideas, one or more specified severe reductions between l,l00C and 950C for low C and low N systems; and a manufacturing method for steel plates and strips having high strength, toughness and weldability, in which the severe reduction in the low temperature region, conventionally considered as a necessary condition to ensure toughness, is made as small as possible. As a result, various defects in the severe reduction in the low temperature region in the conventional method, such as the lowering of the strength, the occurrence of an inferior shape of products and the decrease of productivity due to the necessity of suspending the rolling to lower the rolling temperature, can be eliminated. Its industrial merit is very great.

The reasons of defining each element of the steel composition, and the rolling and coiling conditions in the manufacturing process of this invention will be set forth in detail in the following.

The upper limit of C is defined as 0.15 percent in order to ensure the toughness and weldability, to satisfy the strength as a Nb-steel, to enlarge the possible recrystallization temperature region of austenite in the course of rolling and to ensure a sufficient solubility of Nb as a solid solution at high temperature; and its lower limit is defined as 0.005 percent (desirously 0.01 0.12 percent) in order to ensure the precipitation hardening of NbC.

While Si is effective to improve the strength by hardening, its upper limit is defined as 0.60 percent, considering from the toughness, weldability and particularly the deterioration of toughness of the bond part when used in a welding process using a large heat input.

Mn is added in an amount of at least more than 0.70 percent for the purpose of lowering the transformation temperature of a low C steel, rather than as a necessary component in the steel making process. While Mn itself contributes to improve the toughness of steel through the effect of making fine ferrite grains and to harden the steel through the effect of solid solution hardening and enrichment of the p earlite, the addition of too much thereof enriches the upper bainite structure which causes a deterioration in the yield point and toughness. Therefore, its upper limit is defined as 2.0 percent. A favorable amount of Mn is l .0 2.0 percent.

Less than 0.01 percent of Nb has no effect on the steel. On the other hand, the addition of too much Nb has not only no effect on increasing the amount of Nb dissolved in the austenite as a solid solution at the soaking temperature, but also, has the danger of deteriorating the weldability, particularly of decreasing the toughness of the bond part and of the weld metal in a welding process using a large heat input. Therefore, its upper limit is defined as 0.15 percent.

For steel plates and strips used in a rolled form, the effect of the addition of V both on the strength and toughness is inferior to the effect of Nb. Therefore, V is not used unless its addition is specially necessary. When a definite strength is required and the upper limit of Si, Mn as well as Nb is restricted, up to 0.20 percent of V is added to ensure the strength. Its upper limit is defined from the point of toughness.

A] may be added when it is necessary as a steel making component. Its upper limit is defined as 0.08 percent from the point of toughness and weldability.

N is contained as a unavoidable impurity element in steel. However, as up to 0.0l5 percent thereof has only small influence on the characteristics of the inventive steel, such an amount may be allowable.

While the inventive steel material is of the components above mentioned, suitable amounts of Ni, Cu, Cr, Ti and Zr may be added unless their addition has any influence on the feature of the invention, or for the purpose to supplement the characteristics as will be explained in the following.

Similar to Mn, Cu and Cr has respectively, the effect of decreasing the non-equilibrium transformation temperature during cooling, similar to Mn. As Ni is moreover an element to increase the toughness, less than 1 percent thereof may be added. Each of Cu and Cr gives weather resistance and strength to the steel. Their addition is sufficient within 1 percent, respectively. As Ti has a great affinity with N and fixes N in steel as a nitride, it is an effective element for the inventive low N, Nb-steel. Moreover, when Ti is added, the precipitation hardening due to the precipitation of fine carbides and the improvement of weldability due to the formation of fine nitrides may be expected, and the toughness and ductility in the perpendicular direction of rolling is improved due to the diminution of MnS by the formation of Ti sulfide. Therefore, Ti may be added up to 0.20 percent. As Zr fixes N as nitride and forms sulfide similar to Ti, up to 0.20 percent thereof may be added.

The reason why the heating temperature of the slab before hot rolling is defined as higher than 1,200C is to sufficiently dissolve Nb in the steel as a solid solution and to utilize as much of the Nb as possible for the precipitation hardening of the ferrite matrix, and to avoid the lowering of productivity due to the necessity of lowering the temperature of the heating furnace from a high temperature, such as, 1,230C or 1,300C, for every rolling of Nb-steel. However, when the lowering of a soaking temperature is necessary in relation to the furnace and which includes at least one severe reduction with a draft of more than 26 percent per one pass is given in the temperature range between 1,100C and 950C, is characteristically different from the conventional rolling method of Nb-steel.

While it has been admitted that, in the rolling of Nbsteel, the rolling below 1,050C retards the crystallization, it can be completed in a very short period after the rolling so long as the rolling temperature is not lower than 950C according to the inventive method. Moreover, as already mentioned, the austenite grains obtained are, at the same time, finer than those obtained in the conventional methods.

Table 4 shows the experimental results to indicate the necessity of the inventive severe reduction, i.e., at least one reduction with a draft of more than 26 percent per one pass between l,100C and 950C, to obtain a finish thickness of IO 14.3 mm for the slab with the composition as shown in Table 3. For example, rolling schedule 1 shows that the slab is heated at l,250 1,260C, rolled with a total reduction of 44 percent down to l,l00C, rolled continuously with reductions of respectively 37.5 percent and 31.3 percent between 1,100C and 950C, and then rolled below 950C with a total reduction of 55.2 percent to finish the rolling at 830C.

control of reduction between 1,100C and 950C or T bl 3 below 950C as will be set forth later, the temperature lowering may naturally be adopted so long asthe tern- Composition (an) perature of the heatmg furnace 1n heatmg the 1nvent1ve Steel C S1 Mn (V1 V o I Slab h than 1.7200 I c 0.10 0.24 1.26 0.04 0.04 The 1nvent1ve rolling method, 1n wh1ch the rolling 1s D 0 0 0 I 0 04 0 04 commenced after taking out the slab from the heating Table 4' Rolling Total Amount of each reduc- Total Finish Steel schedule reduction between I 100C reductempetion and 950C tion rature above 76) below (C) 1 100C 950C (7 1) lnven- 4 68 36.3 55.4 s tive method 10 78 no reduction 64.5 840 Comll 78 no reduction 64.7 780 parimethod Rolling Yield Tensile ElongavE vTr vTr schepoint strength tion dule (kglmm (kg/mm (74) (kg-mlcm l (Cl ("Cl Table 4 Continued Rolling Yield Tensile ElongavE., vTr m.

S h point strength tion dule (kg/mm (kg/mm) (kg-ml m l ("6) 10 49.3 60.4 37.5 7.0 l5 -33 ll 48.2 57.3 38.0 9.8 -40 12 39.8 52.l 39.0 14.8 28 48 l3 49.2 60.3 36.5 l0.3 -21 37 I4 47.8 58.2 38.5 13.2 25 -43 Heating temperature: 1250 l260C Finish thickness: l0 l4.3 mm Coiling temperature: 580 620C (In rolling schedule 2, 5. 8 and I2: 700- 750C) The following points can be seen from Table 4.

By comparing No. 1,2, 10, 12, 13 and 14, it is proved that to make the total reduction above l,lOOC as more than 50 percent, for instance, as claimed in Japanese Patent Publication, Sho 44-27139, is not the necessary condition to obtain the result, vTr C, which is one object of the steel in this invention. In No. 1, although the total reduction above l,lOOC is 44 percent, a quite excellent toughness, vTr 58C and vTr 75C, is obtained by the inventive rolling.

On the other hand, while the total reduction above l,lOOC in No. 10 and No. ll is respectively 78 percent, in No. 12 is 73 percent and in No. 13' and No. 14 is respectively 67 percent, their toughness are very inferior, as vTr l5 =28C, as compared with the inventive steels. Further, under a severe rolling at low temperature by lowering the finish temperature as in No. ll and No. 14, the improvement in toughness is only slight. These results show the fact that a severe reduction between l,lOOC and 950C, the firt characteristic of this invention, is the necessary condition to improve the toughness. The reason why the necessary severe reduction between l,lOOC and 950C is defined as more than 26 percent per one pass is that, in they range of the inventive steel, such is the possible reduction to recrystallize austenite completely by the rolling amount down to 950C. The experimental results in Table 4 show that, when at least one reduction with a draft of more than 26 percent per one pass is carried out within said temperature range, a steel having quite excellent low temperature toughness can be manufactured. When the rolling reduction in the low temperature region below 950C is the same, the finer the recrystallized austenite grains just above 950C are, the finer and more uniform the ferrite grains after cooling become, thus the toughness of the product is improved. Therefore, it goes without saying that the greater the number of severe reductions in the high temperature region, the better are the results. The reason why each reduction and total reduction above l,lOOC are not specially defined in that, also in the conventionally adopted rolling schedule, austenite grains just above l,lOOC are sufiiciently fine to obtain still finer recrystallized austenite grains so long as a severe reduction is applied below l,lOOC. When the lowering of productivity is no problem, a steel having excellent toughness can, of course, be produced, by omitting the rolling above l,lOOC, and performing many severe reductions below l,lOOC.

In this invention, for the purpose of giving a still higher toughness, a low temperature rolling with a total reduction of more than 45 percent is carried out below 950C in succession to the severe reduction in the high temperature region, and'the rolling'is finished in the temperature range between 850C and the Ar transformation point. While the object of this procedure is to transform the very fine recrystallized austenite grains obtained by the severe reduction in the tempera ture range between l,lOOC and 950C into a still finer ferrite structure by the rolling in the temperature range where no recrystallization of austenite takes place. This process has additional advantages not expected from the conventional processes. Namely, by use of the specified severe reduction in this invention, it becomes possible to lighten the weight of low temperature rolling, which has been adopted unevitably in order to ensure the toughness in spite of many problems in the quality of the steel, particularly the strength, and in the productivity. As seen from Table 4, the inventive steel has a very high toughness even at a high finishing temperature of 850C, which is remarkably higher than that adopted in the conventional method. This is the second feature of this invention, and thus the waiting time required for lowering of the rolling temperature by. suspending the rolling, which has been conventionally carried out in order to ensure the severe integrated reduction at low temperature, can be omitted completely or at least shortened, and a similar toughness and higher strength can be secured at a higher finish temperature, thereby eliminating the occurrence of products with undesirable shapes and increasing the productivity.

The reason why the total reduction below 950C is defined as more than 45 percent is to satisfy, as shown in Table 6, the object of this invention, vTr -30C.

The reason why the finish temperature of the rolling is defined as below 850C is in order to insure the lowest density of the nucleation site of the ferrite just before the transformation necessary to obtain a fine grain ferrite structure. This low finish rolling temperature should be insured in additon to the total reduction below 950C, to guarantee the high toughness, even when the rolling speed is high. To finish the rolling at a temperature above the Ar transformation point is because rolling in the region of the coexistence of ferrite leaves the worked structure in the steel after cooling, thereby causing a sudden decrease in toughness.

Table 6 shows the mechanical properties of various steel plates and strips. For manufacture, a slab with the composition shown in Table is rolled after heating at 1,250 1,260C, including one reduction with a draft of 30 33 percent between 1,100C 950C and vary ing the rolling schedules below 950C.

Table 5 Composition ("/1) Steel C Si Mn Nb V Table 6 Rolling Total reduction Total reduction Finish schedule below 950C below 850C temperature 41.6 3 845 16 45.6 no 850 17 51.6 no 850 18 56.7 4 845 19 64.5 no 850 56.8 15.4 820 21 56.4 27.8 800 22 56.5 43.0 780 23 64.7 64.7 750 Yield Tensile ElonvE,, vTr, vTr, point strength gation (kg/m (kg/mm") 0) (kg-m/ m") (C) (C) Heating temperature: 1250 1260 C gig-re reduction between lll00C and 950C: with a reduction per one pass 30 Coiling temperature; 580 620C Finished thickness: 11.7 15.2 mm

From Table 6, is it clearly understood that one object of the inventive steel, vTr, 30C, is satisfied when at least one severe reduction with a draft of more than 26 percent per one pass is applied between 1,100C and 950C and the'total reduction thereafter is more than 45 percent, even if the finish temperature is higher than in the conventional method as 850C, and that the toughness converges to a definite value when the total reduction below 950C is more than 55 percent. Thus, the weight of low temperature rolling is lightened as compared with the conventional rolling method. The reason is due to the facts that, in the method of present invention, the austenite grains just before the recrystallization becomes impossible are very fine as compared with those in conventional methods, and there are a sufficient number of nucleation sites of ferrite even if the denisty of the deformation bands introduced by the rolling thereafter is smaller than ever. No deterioration of toughness occurs naturally when the finish temperature of hot rolling is lowered and a severe reduction isappled in the low temperature region, by which the occurrence of undesirable products and the lowering of productivity can not be avoided, as already stated, as in the conventional method. However, as shown in Table 6, so long as the condition of the severe reduction in the temperature range between 1,100C and 950C is applied, not only can a further improvement of the toughness not be attained, but also there is a tendency of decreasing the strength. Particularly, the strength decreases remarkably when the total reduction below 850C exceeds 30 percent. It is preferable that the total reduction below 850C is defined as less than 30 percent from the standpoint of strength and the occurrence of undesirable products. It goes without saying that, when no serious problems in the strength are present, the rolling with a total reduction of more than 30 percent may also be applied below 850C.

While the rolling condition is as explained above, the strength and toughness are alos influenced by varying the coiling condition after finishing the rolling in the case of steel strips.

In the case of coiling the product after rolling, the coiling should be carried out in temperature range between 500C and 680C in order to obtain steel plates and strips having excellent strength and toughness (particularly higher strength), or in the temperature region above 700C in order to obtain the highest toughness in this invention. The object of the former procedure is to make the amount of NbC and NbN precipitated during transformation as small as possible by a rapid cooling after rolling, aiming at strengthening of ferrite, and, as shown in FIG. 2, the strength increases as the coiling temperature is lowered. However, in Nbsteel, the upper bainite structure, being harmful to the toughness, appears when the coiling temperature is too low, and the yield strength decreases as the upper bainite structure is formed. Therefore, the lower limit of coiling temperature is defined as 500C. On the other hand, as the precipitation hardening decreases remark-- ably when the coiling temperature is higher than 680C due to the over aging of the NbC and NbN precipitated, the upper limit of the coiling temperature is defined as 680C. Further, not only the coiling temperature, but also the cooling rate should be taken into consideration. The cooling rate between the finishing temperature and the coiling temperature is desirously of the order in which the upper bainite is not enriched, namely, below 15C per second.

In coiling the steel strips into a coil after rolling, the fluctuation of mechanical properties along the coil length causes problems, and this fluctuation is particularly remarkable in Nb-steel. To prevent this fluctuation, while a method has been proposed to select the coiling temperature as below the Ar transformation point, this procedure is still unsatisfactory. In this invention, rather than this method or in addition to this method, the coiling temperature of the top-end and bottom-end parts are chosen as 30C C higher than the temperature of the coil middle part within said coiling temperature range, and thus the mechanical properties through the whole length of the coil are uniform.

In the better procedure, (coiled above 700C) in order to increase the toughness to a particularly high grade, it is advisable that, instead of said low temperature coiling, the coiling is carried out at a temperature region above 700C. Thus, as shown in Table 4, the toughness of the products under the rolling schedule 2, 5 and 8, coiled at 700 750C, is superior to that is coiled at 580 620C. This may also been understood from FIG. 2. The toughness is improved remarkably by coiling at a high temperature, though the strength dc creases somewhat due to the over-aging of the NbC and NbN, thus insuring a consistently high toughness.

The present invention is applicable not only for a continuous hot strip rolling process, but also for a roll ing process for thick plates, and a process in which the g 13 plate as it stands is cooled directly after the continuous hot rolling.

The details of the invention are as'explained'above,

pnd the examples of this invention will be set forth betoughness steel comprising hot-rolling a steel composit1on composed of:

. 5 The inventive steel has, as compared with conven- 0 005 tional steels, quite excellent strength, toughness and b gg weldabillty, and can be manufactured with a high pro- 070 201% Mn 7 llUCUVIly, and thus the industrial merit of this invention Q01 517 cnlrmous- 10 O to less than 0.20% V, 0 to less than 0.08% Al 13X p A) P g 0 to less than 1.00% Ni. lishlr shown the (IlHflfllCHl comptmitmns. of the ex- 0 m 1 5 than 1,00% Cr, amples of the inventive steel, and Table 8 shows the 0 t l tha 1,00% C summary of the manufacturing conditions and the me- 15 0 to less than 0.20% Ti, and

chanical properties of the products. Steel materials having chemical compositions as in Table 7 were heated to 1,200 1,300C, and the rolling was commenced immediately. At least one reduction with a draft of more than 26 percent per one pass was made between 1,100C and 950C, and the rolling with a total reduction of more than percent of the plate thickness at 950C was applied below 950C. The rolling was finished between 850C and the Ar transformation point, and then the products were coiled (see Table 7). As obvious from Table 8, all the products have quite excellent strength and toughness.

Table 7 0 to less than 0.20% Zn,

with the remainder being iron and unavoidable impurities, and wherein said hot rolling is carried out after heating to a temperature of higher than 1,200C, and during said hot rolling, at least one severe reduction at a rate of more than 26 percent per pass is effected in the temperature range from 1,100C to 950C, and wherein the total reduction below 950C is more than 45 percent of the plate thickness at 950C, and then finishing at a temperature below 850C and above the Ar transformation point.

Chemical composition (70) Steel grade C Si Mn Nb V Ni Cr Cu Ti Zr F 0.13 0.20 1.23 0.04 O 0.05 0.27 1.53 0.04 0.05 H,H' 0.05 0.27 1.35 0.04 0.04 0.31v 1 0.09 0.26 1.32 0.04 0.04 J 0.12 0.31 1.30 0.04 0.04 0.29 K 0.11 0.32 1.32 0.03 0.05v 0.36 0.30 L,L' 0.12 0.32 1.31 0.03 0.05 0.29 M,M' 0.09 0.25 1.28 0.04 0.03 0.02 N,N 0.11 0.30 1.25 0.03 0.04 0.04 0.0 0.11 0.26 1.32 0.04 0.04 0.03 P 0.10 0.24 1.27 0.04 0.04 Q 0.11 0.27 1.22 0.04 0.03 RR 0.13 0.25 0.95 0.04 S 0.15 0.26 1.22 0.04

Table 8 Finish Coil- Plate Yield Tensile ElonvE, vTr, vTr Steel tempeing thickstrength strength ga- (kg-m/ grade rature tempeness (kg/mm (kg/mm tion cm (C) (C) (C) rature (mm) F 850 600 9.5 43.3 59.2 34.0 16.1 87* G 830 600 11.7 55.0 66.6 37.0 18.4 90 H 840 625 11.7 52.7 63.2 38.0 21.5 -77 96 H 820 740 11.7 44.3 56.0 39.5 28.2 102 1 840 610 14.0 49.3 62.3 40.0 17.0 65 83 J 850 625 11.7 47.3 59.6 35.0 16.7 -62 80 K 830 680 11.7 47.0 58.0 36.0 16.8 -65 84 K 830 700 11.7 39.8 51.5 40.0 24.1 72 L 830 680 11.7 43.8 56.0 36.5 12.3 53 70 L 840 710 11.7 37.9 50.1 40.0 20.1 60 78 M 840 610 11.7 48.7 60.4 37.0 19.7 67 83 M 840 720 14.3 40.5 53.4 39.0 27.8 73 92 N 830 590 11.7 49.1 60.7 37.5 24.0 63 85 N 830 730 14.3 42.7 54.2 39.0 26.4 76 95 O 840 600 11.7 47.8 59.4 36.0 14.7 55 73 O 840 730 14.3 40.1 52.1 40.0 20.9 63 80 P 780 600 11 1 47.2 59.1 36.0 16.8 67 83 Q 770 610 11.7 45.8 57.7 37.0 14.7 62 80 R 840 570 9.5 39.2 55.8 38.0 14.1 55* 73* R 830 730 9.5 32.2 49.3 40.0 20.1 65* 83* S 850 610 11.1 42.8 58.0 39.0 17.5 58 75 for suhsizc test pieces with a thickness of9.5 mm

2. The method of claim 1, wherein the total reduction below 850C is less than 30 percent of the platethickness at 850C.

3. The method of claim 1, wherein, after finishing the hot rolling, the product is coiled in the temperature range between 680C and 500C.

UNITED STATES PATENT OFF ICE n CERTIFICATE OF CORRECTION Patent No. 3349209 Dated November 19, 1974 I KEIZO ISHIZAKI, HIROSHI SEKINE,HISASHI GONDO, all

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the heading of the patent, the names of the inventors and the cities should read:

--[75] Inventors: Keizo Ishizaki, Fukuoka-ken;

Hiroshi Sekine,Tokyo; Hisashi Gondo, Chiba-ken; Koji Wada, Chiba-ken; Tsuyos hi Kawano, Chiba-ken; Hidekazu Watanabe, Chiba-ken; Gou fKanbayashi Chiba-ken; Tadakatsu Maruyama, Tokyo, all of Japan-- Signed and sealed this 7th day of January 1975.

(REAL) MCCOY M. GIBSON JR. C. MARSHALL DANN Attesting, Officer Commissioner of Patents F P0405) (10459) I uscoMM-oc eoa1e-peo U. 5 GOVERNMENT PRINTIRG OFFICE 6-360-3, 

1. A METHOD FOR MANUFACTURING A HIGH TENSION, HIGH TOUGHNESS STEEL COMPRISING HOT-ROLLING A STELL COMPOSITION COMPOSED OF: 0.005 - 0.15% C. LESS THAN 0.60% SI, 0.70 - 2.0% MN, 0.01 - 0.15% SI, O TO LESS THAN 0.20% V, O TO LESS THAN 0.08% AL, O TO LESS THAN 1.00% NI, 0 TO LESS THAN 1.00% CR, 0 TO LESS THAN 1.00% CU, 0 TO LESS THAN 0.20% TI, AND 0 TO LESS THAN 0.20% ZN, WITH THE REMAINDER BEING IRON AND UNAVOIDABLE IMPURITIES, AND WHEREIN SAID HOT ROLLING IS CARRIED OTH AFTER HEATING TO A TEMPERATURE OF HIGHER THAN 1,200*C, AND DURING SAID HOT ROLLING, AT LEAST ONE SEVERE REDUCTION AT A RATE OF MORE THAN 26 PERCENT PER PASS IS EFFECTED IN THE TEMPERATURE RANGE FROM 1,100*C TO 950*C, AND WHEREIN THE TOTAL REDUCTION BELOW 950*C, IN MORE THAN 45 PERCENT OF THE PLATE THICKNESS AT 950*, AND THEN FINISHING AT A TEMPERATURE BELOW 850:C AND ABOVE THE AR3 TRANSFORMATION POINT.
 2. The method of claim 1, wherein the total reduction below 850*C is less than 30 percent of the plate thickness at 850*C.
 3. The method of claim 1, wherein, after finishing the hot rolling, the product is coiled in the temperature range between 680*C and 500*C.
 4. The method of claim 2 wherein, after finishing the hot rolling, the product is coiled in the temperature range between 680*C and 500*C.
 5. The method of claim 1 wherein, after finishing the hot rolling, the product is coiled in the temperature range above 700*C. 